Having a large surface area, the micropore structures with a high level of regularity have come into the spotlight in various fields, such as microfluidics, micro-electromechanical systems, biotechnology, organic photo-electronics, etc. For decades, there have been developed different top-down and bottom-up approaches, including etching, colloidal templating, phase separation of block copolymers, emulsification, breath figure (BF), etc., to the synthesis of polymer films with pores of a uniform size from nano to micro scales.
In the typical BF method, a polymer solution is first cast on a substrate under high humidity. During the evaporation of the solvent, the surface temperature of the solution was decreased, which caused the condensation of water as small droplets on the substrate. The ordered water droplets then acts as an ordered template by self-assembly on the surface of the polymer solution. Thanks to its versatility and economic feasibility, the BF method has become the most popular approach to a synthesis of films having a honeycomb-patterned micropore structure. However, the BF method requires the use of equipment for a precise control of the humidity in high level during the synthesis of polymer films and is thus difficult to use in the production of large-area films. In addition, the BF method, particularly in the synthesis of hydrophobic polymer films, further requires the use of nano-particles or surfactants in order to stabilize the condensed water droplets on a polymer solution.
In contrast to the BF method, the non-solvent induced phase separation (NIPS) method enables the production of polymers with micropores on a large scale using a solution under atmospheric conditions. The NIPS method is the method that involves dissolving a polymer in a solvent to form a homogeneous solution, forming the polymer solution into a defined shape and then immersing the formed material in a non-solvent to prepare a separation film. The polymer films, when produced by the NIPS method, have an asymmetric structure with macroviods and thus display a relatively low tensile strength. For this reason, a variety of organic/inorganic additives to enhance hydrophilicity or strengths are commonly added to the polymer solution in the synthesis of polymer films using the NIPS method.
Seeking for a solution to the above-mentioned problems with the prior art, the inventors of the present invention have developed a composite method using a combination of the NIPS and BF methods as disclosed in Korean Patent Publication No. 10-2013-0055413. This method adopts a simple procedure of applying a mixed solution of a solvent and a non-solvent to a polymer film formed on a substrate and then drying the polymer film, thereby making it possible to form a polymer pattern having micropores on different substrates, to prepare a polymer film in the atmospheric environment and to pattern hydrophobic polymers as well as hydrophilic polymers without using a surfactant or an additive.
It is necessary to have the free-standing polymer film peeled from the substrate in order to make the use of the polymer film synthesized on the substrate in the potential applications like tissue engineering materials, separation films, and so forth. The free-standing film is defined as a film a part of which is not in contact with a support material such as a substrate or a wall as in a micropore structure. The polymer film formed on a substrate has such an extremely low thickness from a few micrometers (μm) to a few scores of micrometers (μm). It is therefore required to employ a method of peeling the polymer film from the substrate with more ease, particularly for the fabrication of large-area polymer films. In addition, the polymer films having a through-pore structure in which pores are formed all through the film are of primary interest among the polymer films with micropores. But, there is no known approach to a simple fabrication process for producing hydrophobic polymer films with through-pore structures on a large scale. According to the cited document, Korean Patent Publication No. 10-2013-0055413, it is only in the case of the polymer films in which the polymers are hydrophilic polymers like poly(lactic acid) (PLA) that the micropore pattern can be selectively formed to contain either through (open) pores or blind (dead-end) pores which are interconnected only from one side and terminate inside the film material. In addition, only the blind-pore structure can be formed in the polymer films in which the polymers are hydrophobic polymers, such as polystyrene (PS), polymethyl methacrylate (PMMA), etc.